Radiofrequency (RF) ablation of cardiac and other tissue is a well known method for creating thermal injury lesions at the tip of an electrode. Radiofrequency current is delivered between a skin (ground) patch and the electrode. Electrical resistance at the electrode-tissue interface results in direct resistive heating of a small area, the size of which depends upon the size of the electrode, electrode tissue contact, and current (density). See Avitall B, Helms R. Determinants or Radiofrequency-Induced Lesion Size in Huang S K S, Wilber D J (eds.): Radiofrequency Catheter Ablation of Cardiac Arrhythmias: Basic Concepts and Clinical Applications, 2nd ed. Armonk, N.Y., Futura Publishing Company, Inc., 2000: 47–80. Further tissue heating results from conduction of heat within the tissue to a larger zone. Tissue heated beyond a threshold of approximately 50–55° C. is irreversibly injured (ablated). See Nath S, and Haines DE. Pathophysiology of Lesion Formation by Radiofrequency Catheter Ablation, in Huang S K S, Wilber D J (eds.): Radiofrequency Catheter Ablation of Cardiac Arrhythmias: Basic Concepts and Clinical Applications, 2nd ed. Armonk, N.Y., Futura Publishing Company, Inc., 2000: 26–28.
Resistive heating is caused by energy absorption due to electrical resistance. Energy absorption is related to the square of current density and inversely with tissue conductivity. Current density varies with conductivity and voltage and inversely with the square of radius from the ablating electrode. Therefore, energy absorption varies with conductivity, the square of applied voltage, and inversely with the fourth power of radius from the electrode. Resistive heating, therefore, is most heavily influenced by radius, and penetrates a very small distance from the ablating electrode. The rest of the lesion is created by thermal conduction from the area of resistive heating. See Lin J, Physical Aspects of Radiofrequency Ablation, in Huang S K S, Wilber D J (eds.): Radiofrequency Catheter Ablation of Cardiac Arrhythmias: Basic Concepts and Clinical Applications, 2nd ed. Armonk, N.Y., Futura Publishing Company, Inc., 2000: 14–17. This imposes a limit on the size of ablation lesions that can be delivered from a surface electrode.
Theoretical methods to increase lesion size would include increasing electrode diameter, increasing the area of electrode contact with tissue, increasing tissue conductivity and penetrating the tissue to achieve greater depth and increase the area of contact, and delivering RF until maximal lesion size has been achieved (60–90 seconds for full maturation).
The electrode can be introduced to the tissue of interest directly (for superficial/skin structures), surgically, endoscopically, laparoscopically or using percutaneous transvascular (catheter-based) access. Catheter ablation is a well-described and commonly performed method by which many cardiac arrhythmias are treated. See Miller J M, Zipes D P. Management of the Patient with Cardiac Arrhythmias. In Braunwald E, Zipes D, Libby P (eds): Heart Disease: A Textbook of Cardiovascular Medicine, 6th Ed. Philadelphia, Pa., W.B. Saunders Company, 2001: p742–752. Needle electrodes have been described for percutaneous or endoscopic ablation of solid-organ tumours, lung tumours, and abnormal neurologic structures. See, for example, McGahan J P, Schneider P, Brock J M, Tesluk H. Treatment of Liver Tumors by Percutaneous Radiofrequency Electrocautery. Seminars in Interventional Radiology 1993; 10: 143–149; Rossi S, Formari F, Buscarini L. Percutaneous Ultrasound-Guided Radiofrequency Electrocautery for the Treatment of Small Hepatocellular Carcinoma. J Intervent Radiol 1993; 8: 97–103; and Livraghi T, Goldberg S N, Lazzaroni S, Meloni F, Monti F, Solbiati L. Saline-enhanced RF tissue ablation in the treatment of liver Metastases. Radiology 1995; 197(P): 140 (abstr)].
Catheter ablation is sometimes limited by insufficient lesion size. See de Bakker J M T, van Capelle F J L, Janse M J et al. Macroreentry in the infarcted human heart: mechanism of ventricular tacycardias with a “focal” activation pattern. J Am Coll Cardiol 1991; 18:1005–1014; Kaltenbrunner W, Cardinal R, Dubuc M et al. Epicardial and endocardial mapping of ventricular tachycardia in patients with myocardial infarction. Is the origin of the tachycardia always subendocardially localized? Circulation 1991; 84: 1058–1071. Stevenson W G, Friedman P L, Sager P T et al. Exploring postinfarction reentrant ventricular tachycardia with entrainment mapping. J Am coll Cardiol 1997; 29: 1180–1189. Ablation of tissue from an endovascular approach results not only in heating of tissue, but of heating of the electrode. When the electrode reaches critical temperatures, denaturation of blood proteins causes coagulum formation. Impedance can then rise and limit current delivery. Within tissue, overheating can cause evaporation of tissue or blood water and steam bubble formation (steam “pops”) with risk of uncontrolled tissue destruction or undesirable perforation of bodily structures. In cardiac ablation, clinical success is sometimes hampered by inadequate lesion depth and transverse diameter even when using catheters with active cooling of the tip. See Soejima K, Delacretaz E, Suzuki M et al. Saline-cooled versus standard radiofrequency catheter ablation for infarct-related ventricular tachycardias. Circulation 2001; 103:1858–1862. Theoretical solutions have included increasing the electrode size (increasing contact surface and increasing convective cooling by blood flow), improving electrode-tissue contact, actively cooling the electrode with fluid infusion, changing the material composition of the electrode to improve current delivery to tissue, and pulsing current delivery to allow intermittent cooling.
Needle electrodes improve contact with tissue and allow deep penetration of current delivery to areas of interest. Ablation may still be hampered by the small surface area of the needle electrode such that heating occurs at low power, and small lesions are created.
Additionally, it is desirable to map the electrical activity in the heart before, during or after ablation. If the mapping can be performed with the same catheter used for ablation, the user avoids the need for catheter exchange. Moreover, it is desirable to include a mapping assembly on the catheter comprising a plurality of electrodes that can be used to simultaneously map electrical activity at different positions within the heart to provide more efficient mapping.